Marzia Fantauzzi

Exploiting XPS for the identification of sulfides and polysulfides

The identification of surface sulfide and polysulfide species based on the curve fitting of S2p photoelectron spectra and, for the first time, of X-ray excited S KLL Auger spectra has been performed. The different sulfur chemical states present on the surface (sulfide S2−, central S and terminal S in polysulfide chains) could be unambiguously assigned in the chemical state plot. Sulfur atoms in the central or terminal position, respectively, are found on a line with slope ca. −3 irrespective of the cation indicating similar initial state effects. On the other hand, for a given polysulfide, e.g. K2Sn, sulfur atoms both in central or terminal positions are found on the same line with slope −1 indicating similar final state effects. This behavior can be rationalized with the fact that the negative charge in polysulfide chains is located mainly on sulfur atoms in the terminal position; indeed, sulfur present as central S shows a binding energy shift of −0.6 eV with respect to elemental sulfur (S8), and sulfur in terminal S a shift of −2.4 eV. An application of this approach tested on commercial alkali polysulfides is provided for the curve fitting of SKLL signals and sulfur speciation of three different sulfide minerals enargite (Cu3AsS4), chalcopyrite (CuFeS2) and arsenopyrite (FeAsS). Also for the surface of mineral sulfides, terminal S atoms and central S atoms in the polysulfide chains can successfully be identified.

Combined use of X-ray photoelectron and Mössbauer spectroscopic techniques in the analytical characterization of iron oxidation state in amphibole asbestos

Asbestos fibers are an important cause of serious health problems and respiratory diseases. The presence, structural coordination, and oxidation state of iron at the fiber surface are potentially important for the biological effects of asbestos because iron can catalyze the Haber–Weiss reaction, generating the reactive oxygen species ⋅OH. Literature results indicate that the surface concentration of Fe(III) may play an important role in fiber-related radical formation. Amphibole asbestos were analyzed by X-ray photoelectron spectroscopy (XPS) and Mössbauer spectroscopy, with the aim of determining the surface vs. bulk Fe(III)/Fetot ratios. A standard reference asbestos (Union Internationale Contre le Cancer crocidolite from South Africa) and three fibrous tremolite samples (from Italy and USA) were investigated. In addition to the Mössbauer spectroscopy study of bulk Fe(III)/Fetot ratios, much work was dedicated to the interpretation of the XPS Fe2p signal and to the quantification of surface Fe(III)/Fetot ratios. Results confirmed the importance of surface properties because this showed that fiber surfaces are always more oxidized than the bulk and that Fe(III) is present as oxide and oxyhydroxide species. Notably, the highest difference of surface/bulk Fe oxidation was found for San Mango tremolite—the sample that in preliminary cytotoxicity tests (MTT assay) had revealed a cell mortality delayed with respect to the other samples.

Arsenopyrite and pyrite bioleaching: evidence from XPS, XRD and ICP techniques

In this work, a multi-technical bulk and surface analytical approach was used to investigate the bioleaching of a pyrite and arsenopyrite flotation concentrate with a mixed microflora mainly consisting of Acidithiobacillus ferrooxidans. X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and X-ray-induced Auger electron spectroscopy mineral surfaces investigations, along with inductively coupled plasma-atomic emission spectroscopy and carbon, hydrogen, nitrogen and sulphur determination (CHNS) analyses, were carried out prior and after bioleaching. The flotation concentrate was a mixture of pyrite (FeS2) and arsenopyrite (FeAsS); after bioleaching, 95% of the initial content of pyrite and 85% of arsenopyrite were dissolved. The chemical state of the main elements (Fe, As and S) at the surface of the bioreactor feed particles and of the residue after bioleaching was investigated by X-ray photoelectron and X-ray excited Auger electron spectroscopy. After bioleaching, no signals of iron, arsenic and sulphur originating from pyrite and arsenopyrite were detected, confirming a strong oxidation and the dissolution of the particles. On the surfaces of the mineral residue particles, elemental sulphur as reaction intermediate of the leaching process and precipitated secondary phases (Fe–OOH and jarosite), together with adsorbed arsenates, was detected. Evidence of microbial cells adhesion at mineral surfaces was also produced: carbon and nitrogen were revealed by CHNS, and nitrogen was also detected on the bioleached surfaces by XPS. This was attributed to the deposition, on the mineral surfaces, of the remnants of a bio-film consisting of an extra-cellular polymer layer that had favoured the bacterial action.

MeOx/SBA-15 (Me = Zn, Fe): highly efficient nanosorbents for mid-temperature H2S removal

Zinc oxide/ and iron oxide/SBA-15 composites were synthesized using the innovative Two-Solvents procedure and tested as sorbents for the mid-temperature (300 °C) removal of hydrogen sulphide, and then compared with a commercial unsupported ZnO sorbent. The sulphur retention capacity results showed the superior performance of the iron oxide/SBA-15 composite (401 mg S g−1 Fe2O3) in comparison with the zinc oxide/SBA-15 composite (53 mg S g−1 ZnO), both these sorbents being much more efficient than the commercial sorbent (6 mg S g−1 ZnO). The different sorption behaviour was discussed in terms of the nature of the nanocomposites where: (i) the mesostructure of the support was retained with a high surface area and pore volume; (ii) the zinc oxide phase was incorporated inside the SBA-15 channels as a thin amorphous homogeneous layer while the iron oxide was dispersed in form of small maghemite crystallites; and (iii) significant interactions occurred between the silica matrix and the zinc oxide phase. Remarkable differences in the regeneration behaviour of the exhaust sorbents were revealed by temperature-programmed experiments under an oxidizing atmosphere. After regeneration, the sorption properties of the zinc oxide/SBA-15 composite appeared to be enhanced compared to the commercial sorbent. Incomplete recovery of the sorption activity was observed for the regenerated iron oxide/SBA-15 sorbent, whose performance remained far better than that of the ZnO-based one, either fresh or regenerated. In view of its higher sulphur retention capacity and appropriate regeneration temperature (T ≤ 350 °C), the iron oxide/SBA-15 composite is a promising material for the design of advanced sorbents for a thermally efficient H2S removal process from hot gas streams.

Electronic properties of TiO2-based materials characterized by high Ti3+ self-doping and low recombination rate of electron–hole pairs

Factors tuning the functional performances of the various TiO2-based materials in the wide range of their possible applications are poorly understood. Here the electronic structure of TiO2-based materials characterized by Ti3+ self-doping, obtained by a sol–gel route wholly performed in air at room temperature, is reported. In the amorphous hybrid TiO2–acetylacetonate (HSGT) material the formation of the Ti(IV)–acac complex makes it photoresponsive to visible light and allows us to obtain by means of a simple annealing in air at 400 °C a very stable black Ti3+ self-doped anatase TiO2 nanomaterial (HSGT-400), characterized by an extraordinary high concentration of Ti atoms with oxidation states lower than IV (about 26%), which absorbs light in the entire visible range. The very high stability of HSGT-400 is mainly related to the process, which does not require the use of harsh conditions nor external reducing agents. The electronic structure of HSGT, owing to the presence of the Ti(IV)–acac complex, allows the stabilization of superoxide anion radicals on its surface for a very long time (months) at room temperature. The extraordinary low recombination rate of electron–hole pairs gives to HSGT unusual catalytic performances at room temperature allowing the complete removal of 2,4-dichlorophenol from water in about one hour without any light irradiation. Our results clearly highlight the connection among the production process of TiO2-based materials, their electronic structure and, finally, their functional behaviour.

Nanosized surface films on brass alloys by XPS and XAES

Chemical state identification and quantification based on photoelectron spectra is challenging in the case of copper and zinc and their alloys. In this work an analytical strategy for simultaneous chemical state identification and quantification of copper and zinc chemical states in complex layered systems is presented. This approach is based on the curve fitting of the multicomponent X-ray excited Auger spectra, CuL3M4.5M4.5 and ZnL3M4.5M4.5, that clearly distinguish metallic and oxide components and result in separated ILMM,met and ILMM,ox peak areas. On reference copper and zinc compounds, showing only a single chemical state, the intensity ratio R between photoelectron I2p and Auger intensity ILMM was determined. Rmet was obtained using pure metals and a sputtered brass alloy Cu37Zn. Rox was calculated using the pure oxides. Based on these experimental intensity ratios, R, a quantification factor, k = Rox/Rmet, is calculated for both copper and zinc. This quantification factor k is independent of the instrument employed for the analysis, as proved here by using different spectrometers. The factor k is then used to transfer the experimental Auger intensity ratio (ILMM,met/ILMM,ox) into the I2p,met/I2p,ox intensity ratio, which is required for the quantitative analysis by XPS. The potential of this approach based on XPS and XAES for the patina studies on copper alloys, relevant in various fields including corrosion and cultural heritage, is presented for Cu37Zn model brass alloy after different surface pre-treatments. This approach has proven to be successful.

Electrochemical and XPS surface analytical studies on the reactivity of enargite

Enargite, a copper arsenic sulfide having the formula Cu 3 AsS 4 , is a source of arsenic and may cause environmental problems owing to the release of toxic elements upon oxidation, especially in acid mine effluents. In this work the oxidative dissolution of enargite has been studied on freshly cleaved samples exposed to distilled water, sulfuric acid solution at pH 4 and acidic FeCl 3 or Fe 2 (SO 4 ) 3 solutions at pH ca . 2 with 0.025 M Fe 3+ simulating abiotic acid mine drainage environments. The open circuit potential of the acidic solutions with ferric ions achieved stabilization at + 0.72 ± 0.02 V NHE thus, according to the mixed potential theory, the redox couple Fe 3+ /Fe 2+ strongly polarizes the enargite surface towards positive potentials. Solution analyses showed that in these conditions about 10–14 μg copper are released every 24 h into solution from approximately 0.5 cm 2 enargite surface. Based on the amount of dissolved copper, the thickness of the dissolved enargite has been calculated to be about 60–130 nm. XPS analyses of the reacted enargite surfaces revealed no changes in the binding energy of copper Cu2p3/2 found at 932.4 ± 0.2 eV, arsenic As3d5/2 found at 43.3 ± 0.2 eV and of sulfur, S2p3/2 at 161.9 ± 0.2 eV compared to the pristine surface, whereas a prominent sulfur signal appeared at ca. 163.5 ±0.2 eV, assigned to sulfur in a copper-deficient layer. The XPS quantitative analysis performed by applying a three-layer model revealed the presence of a metal-deficient layer of ca. 0.7 nm thickness on the enargite surface. The interface beneath this layer (estimated thickness 5–10 nm) was slightly enriched in sulfur and depleted in copper. Based on these complementary data from solution analysis and XPS surface analysis, a model similar to the dissolution of binary metallic alloys is here proposed for enargite dissolution under oxidizing conditions.

Physicochemical characterization of metal hexacyanometallate–TiO2 composite materials

The paper describes the synthesis and characterization of novel TiO2–metal hexacyanometallates (MHCMs) composite materials. The starting material, TiO2, was modified by addition of cobalt-hexacyanoferrate (CoHCF) or iron-hexacyanocobaltate (FeHCC) at various concentrations. The resulting composites were characterized as follows: cyclic voltammetry (CV) followed the formation of TiO2–MHCM clusters, TEM micrographs studied their morphology, XAS and XPS data indicated that MHCM bonds to TiO2 through the nitrogen atom of its –CN group and modifies the environment of Ti compared to that of pure anatase. As expected, and confirmed by UV-Vis and XP-valence band data, the electronic properties of TiO2 were substantially modified: the edge in the composite materials shifted by about −2.0 eV relative to TiO2.

Surface Coating from Phosphonate Ionic Liquid Electrolyte for the Enhancement of the Tribological Performance of Magnesium Alloy

A chronoamperometric method has been applied for the growth of a surface coating on AZ31B magnesium alloy, using the imidazolium alkylphosphonate room-temperature ionic liquid 1-ethyl-3-methylimidazolium ethylphosphonate ([EMIM][EtPO3H]) as electrolyte. A surface coating layer is obtained after 4 h under a constant voltage bias of -0.8 V with respect to the standard electrode. The coating nucleation and growth process correlates well with a 3D progressive mechanism. X-ray photoelectron spectrometry (XPS) analysis of [EMIM][EtPO3H] shows new P 2p and O 1s peaks after its use as electrolyte, as a consequence of reaction between the phosphonate anion and the magnesium substrate. Angle-resolved XPS (ARXPS) analysis of [EMIM][EtPO3H] did not show any change in the composition of the surface before and after chronoamperometry, since the sampling depth (1.5 nm at the highest emission angle) is larger than the cation and anion sizes (ca. 7 and 5 Å, respectively). Characterization of the coating was made by scanning electron microscopy (SEM), focussed ion beam SEM, energy dispersive X-ray spectroscopy, XPS, and ARXPS. FIB-SEM shows that the coating presents a mean thickness of 374 (±36) nm and contains magnesium and aluminum phosphates. Linear reciprocating tribological tests under variable load show that the presence of the coating can reduce friction coefficients of the coated AZ31B against steel up to 32% and wear rates up to 90%, with respect to the uncoated alloy.

Enargite by XPS

X-ray photoelectron spectroscopy was used for characterizing the enargite surface. Freshly cleaved samples were analyzed at liquid nitrogen temperature. Enargite is a copper arsenic sulfide of formula Cu3AsS4; it is used as a minor ore of copper. Enargite is a potential source of arsenic and may create environmental problems through the release of toxic elements upon oxidation.